Physiologic Basis for the ECG
PHYSIOLOGIC BASIS FOR THE ECG: PART 1 AND 2
Presenter Information
Jennifer Montemayor, Ph.D.
Position: Professor of Physiology
CLINICAL CONTEXT
Patient Description: 58-year-old female
Chief Complaint: Lightheadedness for one week, feels "heart racing" in chest.
Recent Behavior: Staying up late due to workload.
Past Medical History (PMH): Well-controlled diabetes mellitus.
Physical Examination (P.E.):
Appears anxious.
Pallor and mild diaphoresis.
Cardiac exam reveals an irregularly irregular beat.
Vital Signs:
Temperature: 36.1˚C (97.0˚F)
Respiratory Rate: 22/min
Heart Rate (HR): 142/min
Blood Pressure: 118/55 mmHg
Blood Glucose: 130 mg/dL
Diagnosis: Atrial Fibrillation
Common Clinical & ECG Abnormalities Associated with Atrial Fibrillation
Symptoms: Lightheadedness, palpitations, anxiety, pallor, diaphoresis, tachycardia, borderline hypotension.
ECG Characteristics:
Absence of P waves.
Irregular R-R intervals.
Tachycardia.
Irregularly irregular uncoordinated atrial contractions can lead to tachycardia and stasis of blood in the left atrium, increasing the risk for clot development.
INTRODUCTION
Foundational Basic Science Concepts Needed
To adequately approach the patient case, understanding the following foundational concepts is essential:
What is an ECG?
What information does it provide?
What is a P wave?
What is an R-R interval?
What defines tachycardia?
How is HR determined on the ECG?
OBJECTIVES
Identify ECG waves (P, QRS, T), intervals/segments (P-R, QT, RR, ST), and characterize corresponding electrical events.
Evaluate how waves of depolarization and repolarization are perceived by a recording electrode.
Understand voltage and time calibration on ECG graphs.
Define normal ranges for interval durations: P-R, QT, RR, and the significance of abnormal intervals.
Apply knowledge of lead configuration for limb leads, augmented voltage leads, and precordial leads to understand their "view" in the frontal or horizontal plane.
Determine heart rate from example ECGs.
Classify each of the 12 leads by their "views" of the heart: anterior, left lateral, inferior, right ventricular/upper right.
Determine the mean QRS quadrant/vector from example ECGs.
Evaluate deviations from the normal mean QRS axis and potential anatomical/physiological causes for Left Axis Deviation (LAD) or Right Axis Deviation (RAD).
FOCUSED READING LINKS
Reference reading materials from www.cvphysiology.com for in-depth understanding:
Electrocardiogram-General Description
Electrocardiogram Leads
Volume Conductor Principles and ECG Rules of Interpretation
Mean Electrical Vector
Ventricular Depolarization: Sequence
Mean Electrical Axis
Standard Limb Leads
Augmented Limb Leads
Chest Leads (Unipolar)
OUTLINE
PART 1
I. Introduction
II. ECG waves, intervals, segments
III. Determination of heart rate
IV. Properties of recording depolarization and repolarization
PART 2
V. Standard 12-lead ECG
A. Frontal Plane Leads (6): Standard Limb Leads and Augmented Voltage Limb Leads
B. Precordial Leads (6)
VI. Determination of Mean QRS Axis
Axis deviation and rotation, causes of deviation, and impact on ECG
PART 1
I. INTRODUCTION
Overview of ECG Concepts
ECG measures the average of all electrical potentials generated during electrical depolarization and repolarization in the heart as recorded from the body's surface.
Depolarizing and repolarizing electrical currents are conducted within the heart via conduction pathways and between contractile myocytes via gap junctions.
Electrical activity is projected through body fluids to recording electrodes placed on the surface of the body.
Different ECG leads provide diverse views of the average vectors of cardiac electrical activity.
II. ECG WAVES
A. Waves and Their Meanings
P Wave: Represents atrial depolarization. Sourced from SA node and spreads towards the AV node. Vector moves right to left and slightly inferiorly; normal range is around 0° to 70°.
QRS Complex: Represents ventricular depolarization, average vector of current flow goes from base to apex (R wave). Appearance depends on the view of the ECG lead and individual patient's QRS vector.
T Wave: Represents ventricular repolarization. Average vector flows from apex to base; it is slower than depolarization.
B. Intervals and Segments
PR Interval: Refers to atrial depolarization & AV nodal delay.
QT Interval: Encompasses both ventricular depolarization and repolarization.
ST Segment: Corresponds with ventricular contraction and ejection (isoelectric line).
RR Interval: Used to calculate heart rate.
TP Interval: Indicates ventricular relaxation and filling. Note: indicates mechanical events following electrical events.
C. Time and Voltage Units
Horizontal:
5 large squares = 1 second (5 × 0.2 seconds per large square)
1 mm (small square) = 0.04 s
Vertical:
2 large squares = 1 mV (2 x 0.5 mV per large square)
III. DETERMINATION OF HEART RATE
Normal Heart Rate Range: 60-100 bpm
>100 bpm denotes tachycardia
<60 bpm denotes bradycardia
Common Methods of Calculation
Method 1: Estimate HR Based on Number of Large Boxes Between QRSs. (Note: based on normal paper speed of 25 mm/s)
# Large Boxes
Calculation
HR (bpm)
1
300/1
300
2
300/2
150
3
300/3
100
4
300/4
75
5
300/5
60
6
300/6
50
7
300/7
43
8
300/8
38
9
300/9
33
10
300/10
30
Rationale: Large box = 0.2 seconds.
Method 2: Count R-R Intervals in a 6-second strip and multiply by 10.
Using the tick marks on the EKG page, know that 5 large boxes = 1 s. Count the number of R-R intervals in 6 s intervals. Multiply by 10 to obtain bpm.
IV. PROPERTIES OF ECG RECORDING
Depolarization and Repolarization Principles
Wave of depolarization toward a positive electrode records a positive deflection.
Wave of depolarization moving away from a positive electrode records a negative deflection.
Wave of depolarization moving perpendicular to a positive electrode records no net voltage (isoelectric).
Wave of repolarization moving away from a positive electrode records a positive deflection.
Wave of repolarization moving toward a positive electrode records a negative deflection.
Wave of repolarization moving perpendicular to a positive electrode records a biphasic wave (with a negative deflection preceding a positive one).
PART 2
V. STANDARD 12-LEAD ECG
A. Frontal Plane Leads (6)
Standard Limb Leads (leads I, II, III): Bipolar leads with one positive and one negative electrode.
Augmented Voltage Leads (aVL, aVR, aVF): Unipolar leads with one positive and two negative electrodes.
B. Transverse Plane: Precordial Leads (6)
Chest Electrodes: Unipolar leads (V1-V6), with chest electrodes being positive and body as the common ground.
Orientation:
Right ventricle lies anteriorly and medially; left ventricle lies posteriorly and laterally.
R-Wave Progression
Normal R-Wave progression indicates increasing R-wave amplitude from right to left across the precordial leads:
V1 generally records the smallest R wave;
V5 or V6 typically record the largest R wave.
Transition zone where QRS switches from mostly (-) to mostly (+) is generally at either V3 or V4.
12-Lead Configuration Details
Limb Leads: 3 standard leads, 3 augmented leads (4 electrodes)
Precordial Leads: 6 electrodes for chest placement
Lead Configurations: Understanding the angle of orientation is crucial (e.g., for leads I, II, III, and aVL, aVR, aVF).
VI. DETERMINATION OF A PATIENT'S MEAN QRS AXIS
Mean Electrical Axis Determination
QRS recorded by different leads helps evaluate the mean current flow.
Understanding how to determine the QRS would involve:
Perpendicular leads yield isoelectric voltage.
Parallel leads yield large positive or negative voltage depending on the direction.
Normal Mean Electrical Axis Frontal Plane:
4 quadrants system from 360° axial reference.
Normal quadrant defined as +90° to 0°; boundaries set by leads aVF and I.
Causes of QRS Axis Deviation
Left Axis Deviation (LAD):
Common causes:
Physical shift (e.g., pregnancy, obesity).
Left ventricular hypertrophy due to systemic hypertension or aortic valve issues.
Infarction of the right ventricle.
Right Axis Deviation (RAD):
Common causes:
Physical shift (e.g., tall and lean individuals).
Right ventricular hypertrophy due to pulmonary valve issues or pulmonary hypertension.
Infarction of the left ventricle.
Understanding QRS Axis Representation
One method to identify mean QRS vector involves finding the most isoelectric lead and rotating 90° to the quadrant identified.
Changes associated with axis deviation from normal mean electrical axis can indicate changes in heart structure or function.
V. SUMMARY
ECG Waves, Intervals, Segments: Includes identification of P, QRS, T waves and intervals.
Normal Interval Durations:
PR: 0.12 - 0.20 s,
QRS: ≤ 0.10 s.
Heart Rate Determination Methods: Based on counting large boxes or R-R intervals.
Coupling of ECG Properties: Relate to depolarization, repolarization recordings, and understanding of lead configurations.
QRS Axis: The identification of quadrants and potential causes of deviations is essential in interpreting ECGs effectively.
Note: The summary emphasizes a broad understanding of the ECG principles and their clinical significance, focusing on recognizing and interpreting reliably the patterns observed in ECGs and their implications in clinical practice.